It is widely held that healing and/or structural correction is greatly facilitated when a bone is stabilized in the proper position. Various devices for stabilization of bone are well known and routinely practiced in the medical arts. For example, an abnormal spine can be stabilized using a substantially rigid or semi-rigid interconnecting means (rod or plate) and fastening means (screws, clamps, hooks, claws, anchors, or bolts). Multiple fasteners are placed into the spinal pedicle of each vertebra and linked by at least one interconnecting means. Once in place, these systems substantially immobilize the spine and promote bony fusion (arthrodesis).
With respect to the thoracic spine, it may be afflicted with a variety of ailments, some so severe as to require surgical intervention. A disc herniation may compress the spinal cord and/or nerve roots and cause pain, loss of function, and even complete paralysis of the legs with loss of bowel and bladder control. The correct treatment for such conditions is the removal of the offending discal tissue. However, this has proven both difficult and quite dangerous. When the discs of the thoracic spine are approached posteriorly (from behind), the spinal cord is in the way. To approach the same herniation anteriorly (from the front) requires the very formidable procedure of thoracotomy (cutting open the chest) and moving the heart and lungs out of the way.
Quite recently surgeons have begun performing these procedures from a lateral approach to the spine (from the side) using fiber optic viewing instruments called thorascopes and numerous small surgical openings through the chest wall (portals) through which various surgical instruments, such as burrs, rongeurs and curettes, may be placed to remove these disc herniations while avoiding formal thoracotomy. Because the discs are very narrow in the thoracic spine and the surgeon is approaching the spine laterally, there is very little space in which to work as the disc is entered. Therefore, the amount of disc removal may be limited. Alternatively, the surgeon might remove the pedicle to gain access to the spinal canal risking further weakening of the already diseased area.
Sometimes, for a variety of reasons, including the removal of disc material, the thoracic spine may become unstable (too much motion) at any given level. Historically, this has been treated by fusion, the joining together permanently of the unstable vertebrae via a bridge of bone so as to eliminate all motion at that location. Fusions about the thoracic spine have been performed either anteriorly or posteriorly, either procedure being a serious surgical undertaking.
Stability of the spine is required for fusion to occur. For this reason, and for the purpose of correcting spinal deformity, it is often necessary to use hardware to rigidly internally fixate (stabilize) the spine. To date, the only benefit the use of the thorascope has provided in this regard is to allow the previous thoracotomy incision to be somewhat smaller.
Thus, the prior art includes numerous drawbacks which have not been entirely addressed. Traditionally, the surgical techniques for stabilization of bone required large incisions (upwards of 6 cm in length) and a considerable amount of muscle be cut and stripped away (retracted) from the bone for an “open” visualization of the bone and access thereto for the placement of the fasteners and instrument implantation. Although this so-called “open” surgical technique has successfully treated non-unions, instability, injuries and disease of the spine, it is not without disadvantages. Given the invasive nature of this technique, a lengthy healing time and considerable post-operative pain for the patient is common.
With respect to the human lumbar spine, the treatment of discal disease with neural compression has generally been from a posterior (from behind) approach. Lumbar discs are generally quite large and it is only those protrusions occurring posteriorly which compress the neural elements, which are themselves posterior to the discs. These posterior approaches have included both true posterior approaches and posterolateral approaches to the discs. Further, such approaches have been made via open incisions or through percutaneous stab wounds. In the latter case, instruments are inserted through the stab wounds and monitored by the use of radiographic imaging or the use of an endoscopic viewing device. While it is possible to also decompress a posterior disc herniation in the lumbar spine from an anterior approach (from the front), doing so requires the removal of a very substantial portion or all of the disc material in the front and mid portions of the disc, thus leaving that disc and that spinal segment generally unstable. Therefore, such an anterior approach to the lumbar spine has been reserved for those instances where a fusion is to be performed in conjunction with, and following such a disc removal.
Fusion is generally induced with the application of bone or bone like substances between bones to induce bony bridging; such procedures have been performed outside the vertebral bodies and/or between the vertebral bodies, the latter being known as an interbody fusion. Such interbody fusions have been performed from posterior, posterolateral and anterior. Interbody fusion from the posterior approach, while still in use, has been associated with significant complications generally related to the fact that the delicate dural sac and the spine nerves cover the back of the disc space and are thus clearly at risk for damage with such an approach. The posterolateral approach has generally been utilized as a compliment to percutaneous discectomy and has consisted of pushing tiny fragments of morsalized bone down through a tube and into the disc space.
In anterior interbody spinal fusion, the path of entry of the fusion material into the intervertebral space is performed from a straight anterior position. Such an anterior position is achieved in one of two ways. First, by a straight anterior approach which requires that the peritoneal cavity, which contains the intestines and other organs, be punctured twice, once through the front and once through the back on the way to the front of the spine; or secondly, by starting on the front of the abdomen off to one side and dissecting behind the peritoneal cavity on the way to the front of the spine. Regardless of which approach to the front of the spine is used, and apart from the obvious dangers related to the dense anatomy and vital structures in that area, there are at least two major problems specific to the anterior interbody fusion angle of implant insertion itself. First, generally at the L.sub. 4 and L.sub. 5 discs, the great iliac vessels bifurcate from the inferior vena cava and lie in close apposition to and covering that disc space, making fusion from the front both difficult and dangerous. Secondly, anterior fusions have generally been done by filling the disc space with bone or by drilling across the disc space and then filling those holes with shaped implants. As presently practiced, the preferred method of filling the disc space consists of placing a ring of allograft (bone not from the patient) femur into that disc space. An attempt to get good fill of the disc space places the sympathetic nerves along the sides of the disc at great risk. Alternatively, when the dowel technique is used, because of the short path from the front of the vertebrae to the back and because of the height of the disc as compared to the width of the spine, only a portion of the cylindrical implant or implants actually engage the vertebrae; thus compromising the support provided to the vertebrae and the area of contact provided for the fusion to occur.
There is, therefore, in regard to the lumbar spine, a need for a new method and apparatus for achieving interbody fusion which avoids the problems associated with all prior methods, and which have included, but are not limited to, nerve damage when performed posteriorly, or the need to mobilize the great vessels when performed anteriorly. Further, the size of the implants are limited by the dural sac posteriorly, and the width of the spine and the delicate vital structures therewith associated anteriorly. Such a method and apparatus for interbody fusion should provide for optimal fill of the interspace without endangering the associated structures, and allow for the optimal area of contact between the implant or implants and the vertebrae to be fused. The method and apparatus should also provide controlled distraction of the bony structures while also providing an interlocking connection to the bony structures to prevent movement or dislodgement of the apparatus.